This invention relates, in general, to encapsulating a workpiece, and more particularly, to an apparatus for and a method of transferring an encapsulating material to a mold cavity.
Packaging protects semiconductor devices from deleterious environmental effects including moisture, corrosion, and mechanical stresses and damage. A semiconductor device and electrically coupled leadframe are placed in a cavity of a first plate of a mold apparatus for the packaging process. When the mold apparatus is closed, the cavity is sealed by pressing the first plate and a second plate tightly against the leadframe around a perimeter of the cavity. An encapsulation material in a reservoir or pot of the mold apparatus is liquefied, forced from the pot through a groove or runner and into the cavity, and hardened to encapsulate the semiconductor device.
During the packaging process, the encapsulation material leaks from the runner and forms undesired thin sheets or webs, known as flash, on the molding plates themselves and on ends of electrical leads of the leadframe outside the cavity. Removal of the undesired flash is both time consuming and expensive. The removal process can destroy the electrical leads rendering the packaged semiconductor device useless and can also destroy the high-priced molding plates.
To prevent the deleterious formation of flash, the pot, the runner, and the cavity containing the semiconductor device must all be properly sealed during the packaging process. Extra force is applied in the runner region of the first and second mold plates to ensure proper sealing of the runner. However, flash formation still originates from the runner when packaging leadframes that are too thick. When packaging leadframes that are too thin, flash formation originates from the cavity. Therefore, different sets of molding plates are used for packaging leadframes of different thicknesses.
Due to the high temperatures of the packaging process and the alignment precision required for the molding assembly, changing the molding plates for leadframes of different thicknesses is a time consuming process. Due to the extreme precision required for machining the molding plates which cost approximately $85,000 per set, the semiconductor device packaging process is an expensive one to maintain, especially when flash problems are common or when leadframes of different thicknesses need to be packaged. Furthermore, the actual thickness of a leadframe for a specified dimension will still vary, and thus the use of different molding plates does not provide a panacea for the problem of flash formation.
Accordingly, a need exists for eliminating flash formation resulting from the leakage of encapsulation material from the runner and from the cavity during the semiconductor device packaging process. Additionally, a need also exists for reducing the capital cost of maintaining different sets of molding plates for packaging leadframes of different thicknesses. The method and apparatus for satisfying these needs should be more economical and less time consuming compared to the old methods and apparatuses.